Vegetation dynamics and disturbance history in three deciduous forests in boreal Sweden

Abstract. Remaining deciduous forests in the Fennoscandian boreal landscape have high ecological value, and are considered as key components of the forest landscape as well as remnants of a former natural forest type. To improve our understanding of the formation of deciduous forests, we studied past disturbance regimes and vegetation dynamics in three deciduous forests in boreal Sweden using dendro‐ecology, pollen analysis and charcoal analysis. We identified three stages in the development of the studied stands. Firstly, the coniferous period (pre 1800), a long‐lasting period characterized by frequent fires, livestock grazing and extensive agriculture during which Pinus sylvestris was dominant. Secondly, the transformation period (1800 ‐ 1900), when logging removed most pines from the sites while fire and grazing continued. At the time of the last fire, the sites lacked a local seed source of pines, resulting in a post‐fire succession dominated by deciduous species with the capacity to disperse over long distances. Thirdly, the deciduous period (1900 ‐ present), with little or no disturbance from fire, grazing or logging. Thus, the present deciduous stands have their origins in a complex interaction between changes in fire regime, extensive land use patterns and logging, contrary to earlier simplified explanations. We conclude that the complexity of historical patterns of land use, vegetation dynamics and disturbance should be acknowledged in the future when selecting areas for nature conservation and developing models for ecologically oriented forestry.     

白桦成熟合子胚快速不定芽诱导体系的建立

以白桦成熟合子胚为外植体,建,3-7白桦合子胚快速不定芽诱导体系：合子胚经消毒纵切后,在培养基WPM＋2．0mg·L-16-BA＋0．2mg·L-1NAA中,可直接诱导形成不定芽,诱导率最高为93．3％,不定芽诱导数为8．7个;WPM＋4．0mg·L-1TDZ上也有较好的诱导效果,不定芽诱导率为90．2％,不定芽诱导数为7．6个,这表明WPM＋2．0mg·L-1 6-BA＋0．2mg·L-1 NAA是白桦成熟合子胚不定芽诱导的较优培养基。NAA能有效促进嫩茎生根,其最佳浓度为0．2mg·L-1。     

The content of total sulphur and sulphur forms in birch (Betula pendula Roth) leaves in the air-polluted Krusne hory mountains

In leaves of birch (Betula pendula Roth), changes in the content of total sulphur and its inorganic and organic forms were determined in relation to the decreasing air-pollution load (SO₂) in the air-polluted Krusne hory mountains and the Decin sandstone highlands in 1995, 1998, 2001 and 2004. Results have shown that birch is able to use considerable amounts of sulphur taken through leaves from air-pollution load. Birch responds fast to changes in air-pollution load by fall in the content of total and inorganic forms of sulphur in leaves.     

Interactions between mountain birch seedlings from differentiated populations in contrasting environments of subarctic Russia

So far very few experiments have accounted for the combined effect of two phenomena co-occurring in stress gradients: local adaptation to stress and the increase in facilitation with increasing stress (predicted by the stress-gradient hypothesis, SGH). Mountain birch (Betula pubescens subsp. czerepanovii) facilitates conspecific seedlings in subarctic high stress sites and is capable of rapid evolutionary adaptation, being therefore a good model species for a study combining local ecotypes and SGH. A within-species experiment was conducted to test SGH in three stress gradients, detect potential local adaptations between low and high stress populations, and assess their effects on seedling-seedling interactions. Although no evidence for local adaptation was detected, high and low stress populations showed some differentiation, possibly explained by decreasing phenotypic plasticity in high stress conditions and/or neutral evolutionary mechanisms. Weak support for SGH was detected. While facilitation was unaffected by seedling origin, low stress populations showed better competitive ability.     

Impact of tree species on nutrient and light availability: evidence from a permanent plot study of old-field succession

This paper compares vegetation composition, light availability, carbon and nutrient pools and Ellenberg indicator values among four old-field successional permanent plots that have received an initial treatment (ploughing, herbicide or sterilisation) prior to being left undisturbed in 1969, a second set of six plots received additional treatments (continued ploughing or mulching until 1982). On all plots species rich pioneer forests developed. Vegetation still varies among plots with different initial treatments: Sterilised plots can be distinguished from the others by dominance of Betula pendula, ploughed plots by Fraxinus excelsior, whereas herbicide-treated plots are intermediate with proportions of both species. By affecting light availability at the ground, tree species in turn influences ground vegetation and soil properties. Light availability turned out to be the dominant factor determining the composition of the vegetation in old-field succession.     

Shrub expansion may reduce summer permafrost thaw in Siberian tundra

Climate change is expected to cause extensive vegetation changes in the Arctic: deciduous shrubs are already expanding, in response to climate warming. The results from transect studies suggest that increasing shrub cover will impact significantly on the surface energy balance. However, little is known about the direct effects of shrub cover on permafrost thaw during summer. We experimentally quantified the influence of Betula nana cover on permafrost thaw in a moist tundra site in northeast Siberia with continuous permafrost. We measured the thaw depth of the soil, also called the active layer thickness (ALT), ground heat flux and net radiation in 10 m diameter plots with natural B. nana cover (control plots) and in plots in which B. nana was removed (removal plots). Removal of B. nana increased ALT by 9% on average late in the growing season, compared with control plots. Differences in ALT correlated well with differences in ground heat flux between the control plots and B. nana removal plots. In the undisturbed control plots, we found an inverse correlation between B. nana cover and late growing season ALT. These results suggest that the expected expansion of deciduous shrubs in the Arctic region, triggered by climate warming, may reduce summer permafrost thaw. Increased shrub growth may thus partially offset further permafrost degradation by future temperature increases. Permafrost models need to include a dynamic vegetation component to accurately predict future permafrost thaw.     

子午岭林区白桦-辽东栎混交林光合生理生态特征研究

对黄土高原子午岭次生林区白桦林、辽东栎林和白桦-辽东栎混交林3种林分的土壤物理特性和叶片光合特性进行了研究。结果表明:(1)白桦-辽东栎混交林地的土壤水分明显改善,其土壤容重最小、土壤孔隙度最大,且均优于纯林,即混交林地有深层次的土壤水分可供利用,并改善了土壤的物理结构;(2)辽东栎林的光合速率和气孔导度最大,其次为白桦-辽东栎混交林,水分利用率(WUE)为混交林白桦>混交林辽东栎>辽东栎林>白桦林;(3)混交林中白桦、辽东栎的Fv/Fm和Fv/Fo值均较大,与纯林差异不显著;白桦林和辽东栎林的qP和NPQ值均大于混交林。     

濒危植物盐桦离体组织培养特性的研究

目的:探讨新疆濒危植物盐桦离体组织培养的特性。方法:从盐桦原生地阿尔泰阿拉哈克盐湖边采摘盐桦休眠实生苗上的落叶枝条,待其萌发后分别取带芽嫩茎、嫩茎茎段及嫩叶芽尖三种不同材料接种于启动培养基,比较三种盐桦离体组织的诱导分化,继而设计不同激素、不同水平的单因子试验和正交试验,筛选适宜盐桦外植体芽增殖和生根的分化培养基。结果:诱导盐桦芽增殖的最佳外植体是带芽嫩茎,盐桦外植体增殖、壮苗最适培养基为:MS 6-BA 1.0mg/L IBA 0.5mg/L;盐桦外植体生根最适培养基为:1/2MS IBA 0.5mg/L 蔗糖30g/L 琼脂7% 暗光处理3d。结论:本研究筛选获得适宜盐桦芽增殖和生根的最佳培养条件,为高效扩繁和保存盐桦种质资源奠定了基础。